Nanoemulsions have many physical properties that distinguish them from other emulsions. Due to their small mean droplet size, which is often smaller than optical wavelengths of the visible spectrum (thus less than about 400 nm), nanoemulsions usually appear transparent or translucent to the naked eye, even at high droplet volume fractions. The terms sub-micron emulsion (SME) and miniemulsion are sometimes used as synonyms for the term nanoemulsion. Nanoemulsions have great potential for use in many industries and applications.
A nanoemulsion may be defined as a type of emulsion wherein the dispersed/discontinuous phase has a mean droplet size of less than 1000 nm; the components of the continuous and dispersed/discontinuous phases must be immiscible enough to allow for the respective phase formation. Some nanoemulsions may have a smaller range for mean droplet size specified, and it is possible to have more than one dispersed/discontinuous phase. These emulsions are typically composed of a nonpolar phase (usually denoted as the oil phase), a polar phase (typically aqueous and denoted as the aqueous or water phase), a surfactant and optionally one or more additional co-surfactant(s). There may be a narrow droplet size distribution depending on the preparation process.
Nanoemulsions are usually stable against sedimentation or creaming, with high kinetic stability, probably because Brownian motion and diffusion rates are higher than the sedimentation or creaming rates induced by gravity. However, they are usually non-equilibrium systems (typically requiring energy input for formation), and thus thermodynamically unstable, and therefore have a tendency to separate into the constituent phases.
In general, there are two primary methods to prepare a nanoemulsion: (1) by “persuasion” and (2) by “brute force”, which are described in the chapter, “Nanoemulsions”, by Salager, Forgiarini and Marquez, in Pharmaceutical Emulsions and Suspensions, (2nd edition), Nielloud and Marti-Mestres editors, Taylor and Francis (London). Preparation by persuasion involves taking advantage of certain phase transitions, while preparation through brute force involves imparting sufficient shear to reduce the droplet size of the immiscible internal phase below 1000 nm. These methods may be further described as follows.
(1) By Persuasion:
(1.1) Phase Transition from Near-Optimum State Via Change in Single Variable
This method involves change in one formulation variable such as salinity or temperature for a system near optimal (HLD (hydrophilic lipophilic deviation) near 0), such as applying a higher temperature to a Winsor III microemulsion (a middle-phase microemulsion in equilibrium with two excess phases, water and oil; it can be understood as an accumulation of swollen micelles, so numerous that they touch one another, forming a perfectly bicontinuous structure).
(1.2) Phase Transition from Near-Optimum State Via Change in Multiple Variables
This method involves change in more than one formulation variable, such as applying higher temperature and inclusion of additional salt in a Winsor III microemulsion.
(1.3) Catastrophic Inversion
This method involves causing a low internal phase emulsion to invert such that the internal phase becomes the external phase.
(1.4) Phase Transition Stabilized by Liquid Crystal Formation
This method involves stabilization of nanodroplets by liquid crystal formation from a state near HLD=0.
(2) By Brute Force:
This method may involve the use of a high speed mixer, a high pressure homogenizer, a high frequency ultra-sonic device, a small pore membrane, etc.
Surveys of recent literature show that formation of O/W and W/O nanoemulsions by dispersion or high-energy emulsification methods is apparently fairly common, while nanoemulsion formation by condensation or “low-energy” emulsification methods, which take advantage of the physicochemical properties of these systems based on the phase transition that takes place during the emulsification process, is apparently starting to attract interest. The latter mentioned “low energy” procedures can be carried out by operating in particular areas of the phase diagram with a very low interfacial tension, which are areas of liquid crystals and microemulsions; at the end of the emulsification process, nanoemulsions formed are not in thermodynamic equilibrium as was the original system.
Properties of nanoemulsions, such as small droplet size, relative high kinetic stability and optical transparency seem to depend (at least in many cases) not only on composition variables but also on preparation variables such as emulsifying path, degree of mixing energy input and emulsification time. These are described further in T. G. Mason, J. N. Wilking, K. Meleson, C. B. Chang, and S. M Graves, “Nanoemulsions: Formation, Structure, and Physical Properties”, Journal of Physics: Condensed Matter, 18 (2006) R635-R666; and in C. Solans, J. Esquena. A. M. Forgiarini, N. Uson, D. Morales, P. Izquierdo, and N. Azemar, “Nanoemulsion: Formation, Properties and Applications” in D. Shah, B. Moudgil, K. L. Mittal (Eds.), Adsorption and Aggregation of Surfactants in Solution, Surfactant Science Series, Marcel Dekker, New York, 2003, pp 525-554.
Nanoemulsions are an emerging technology which show promise for application in many industrial areas. Current main applications of nanoemulsions seem to concentrate in high value-added fields, such as in nanoreactors for polymerization, in chemical, cosmetic and pharmaceutical applications and in the food industry. One recent example of this relates to a composition and methods for making and using nanoemulsions as a delivery system, the emulsions created by high shear stress technology, and gives applications in the nutritional, pharmaceutical and cosmetics fields.
It is of note that nanoemulsions can typically be formulated using less surfactant than is required for many microemulsions; thus, it is very likely that nanoemulsions will play an increasingly important role commercially. A major cost component involved in producing nanoemulsions is usually the energy input required; methods of reducing this energy cost are of interest.
A recently conducted literature survey revealed no mention of the use of nanoemulsions in drilling, completion, remediation or many other oil field fluids or processes. A few items of interest were noted.
One such item is WO 2005/090851 which relates to the use of polymeric nanoemulsions as drag/friction and/or pressure reducers for multiphase flow. The polymeric nanoemulsions are said to facilitate flow and reduce drag and friction in multiphase pipelines containing both oil and water including oil/water, oil/water/gas, oil/water/solids, and oil/water/gas/solids, such as are used for oil or gas production, gathering, and transmission and for hydro-transport of oil sand or heavy oil slurries. The polymeric nanoemulsion drag reducers are made by combining components with sufficient mixing to form droplets of acceptably small size with the nanoemulsions reported to be storage stable and to have a low viscosity of about 200 cP or less which enables easy pumping. These nanoemulsions have a hydrocarbon external phase, droplets of an aqueous internal phase having water-soluble polymer dissolved therein, where the droplets have an average size below about 300 or even 200 nm, and at least one surfactant. An example of a suitable drag reducing polymer used here is polyacrylamide. One particular application of the composition is the continuous injection of the nanoemulsion polymer product through a subsea umbilical into multiphase flowlines to achieve increased production and/or reduction in pressure drop through the treated system.
Another item noted is U.S. Patent Application Publication No. 2008/0110618 which discloses nanoemulsion, macroemulsion, miniemulsion and microemulsion systems with excess oil or water or both (Winsor I, II or III phase behavior) or single phase microemulsions (Winsor IV) that improve the removal of filter cakes formed during hydrocarbon reservoir wellbore drilling with oil based muds. The macroemulsion, nanoemulsion, miniemulsion and microemulsion systems with excess oil or water or both or single phase microemulsion removes oil and solids from the deposited filter cake. In one embodiment, the emulsion system (a single phase microemulsion, nanoemulsion, or other emulsion) may be formed in situ (downhole) rather than produced or prepared in advance and pumped downhole. Skin damage from internal and external filter cake deposition might be reduced using these systems.
Given the short supply of energy in the world today, there is always a need to produce oil and gas and related materials more efficiently. There are many sources of inefficiency; for example, it is well known that friction pressure losses and subsidence of solid weighting materials reduce such efficiency and that the use of certain additives can increase it; some of this is described in more detail below.
Numerous drilling fluids and procedures used in the drilling of subterranean oil and gas wells along with many related fluids and procedures in the oil and gas industry (such as completion and remediation) are known in the art. Much of the discussion that follows on drilling can be applied to other oil field fluids and processes as well.
In rotary drilling, there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are typically suspended in water or brine. Oil can be emulsified in the water which is the continuous phase. Brine-based drilling fluids are water-based muds (WBMs) in which the aqueous component is brine. Oil-based muds (OBM) are the opposite or “inverse”; solid particles are typically suspended in oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds may be either all oil or water-in-oil emulsions, which are also called invert emulsions. In oil-based mud, the oil can consist of any oil that may include, but is not limited to, diesel, mineral oil, esters, or olefins. OBMs as defined herein include synthetic-based fluids or muds (SBMs) which are synthetically produced rather than refined from naturally-occurring materials. SBMs often include, but are not necessarily limited to containing, olefin oligomers of ethylene, esters made from vegetable fatty acids and alcohols, or ethers and polyethers made from alcohols and polyalcohols, paraffinic and other natural products and mixtures of these.
Solid particles are often added to drilling fluids for various reasons. Weighting agents such as barite, calcite or hematite particles may be added to the drilling mud to increase the density of the fluid and ensure that the fluid provides adequate hydrostatic pressure in the wellbore. These particles may settle and/or stratify in the fluid as it is being pumped through the wellbore. It is well known that settling and sagging of solids such as barite may lead to safety and operational problems, particularly in inclined boreholes. In weighted drilling muds, barite as well as other weighting agents tends to segregate slowly, settle in the lower side of the borehole and start sliding in boreholes drilled at high angles from the vertical. The main problems caused by this phenomenon are pressure control due to density variations or non-linear hydrostatic pressure gradients, lost circulation, high torque and drag. Stuck pipes and plugged boreholes and even lost circulation occur because of the presence of thick and tight barite beds. Traditionally, organophilic clays have been added to drilling fluids to overcome the sag problem. However, these materials increase the viscosity of the drilling mud causing a decrease in drilling efficiency since relative high pumping pressure may be required.
Another important oil and gas operation is completion. Completion is the operation that prepares a well bore for actually producing oil or gas from the reservoir. The goal of the completion operation is to optimize the flow of the reservoir fluids into the well bore, up through the producing string, and into the surface collection system. The nature of the reservoir helps determine the type of completion to be used, such as open hole or cased hole completion, as well as helping determine fluid selection. Many applicable methods and fluids are described in the art.
Oil and gas well remediation is important to the industry as well. This refers to attempts at restoration of the initial characteristics of producing formation rocks or removal of formation damage. Remediation may involve use of one or more of a variety of fluids and methods of application with many described in the art.
There are other important oil and gas operations, including acidizing, stimulation and fracturing. Many fluids and procedures for these are known in the art.
There is still a need for improved methods and compositions for these various operations, and nanoemulsions are found to fill this need.